Certain emails sent to contacts, especially those outside the internal Exchange/non-exchange come in as plaintext with a winmail.dat format.
Sending HTML sources from 3rd party services like gmail do not cause the plaintext / HTML stripping whereas emails from your exchange server do.
There is a message format type of MAPI RTF that works seemingly fine from exchange-to-exchange communications, but can cause issues with some other types of servers — in our case IMAIL server.
In our case, these were Exchange Contacts causing the issue — in which by default had a setting of Use MAPI Rich Text Format of “Use Default Settings”. Furthermore, viewing the contact properties in Outlook showed a setting of Internet Format set to “Send using Outlook Rich Text Format” — changing the latter setting to “Let Outlook decide […]” fixed the issue as far as HTML test emails.
There are also Hub Transport settings for “foreign domains” which can be played with to disable RTF formatting writ large, but this is obviously subject to testing. At the very least, when creating mail contacts for foreign SMTP addresses, particularly non-exchange ones, this is something to consider.